Doctors in some U.S. hospitals prescribe three times as many antibiotics as doctors in other hospitals, putting patients at risk for future super-resistant infections, the U.S. Centers for Disease Control and Prevention ( CDC) warned Tuesday.

In addition, about one-third of the time, prescribing practices to treat urinary tract infections and prescriptions for a common antibiotic called vancomycin included a potential error, for instance, given without proper testing or evaluation, or given for too long a period, the CDC said in its new Vital Signs report.

"Prescribing varies widely among hospitals," CDC Director Tom Frieden told a press telebriefing. "Practices that are not optimal are putting patients at unnecessary risk of future drug-resistant infections, allergic reactions, and intestinal infections that can be deadly."

The CDC study, based on data from 323 hospitals in the United States, found more than half of all hospitalized patients received an antibiotic at some point during their hospital stay.

The most common types of infections for which doctors wrote antibiotic prescriptions were lung infections (22 percent), urinary tract infections (14 percent), and suspected infections caused by drug-resistant Staphylococcus bacteria, such as MRSA (17 percent), it said.

Although antibiotics save lives, they can also put patients at risk for a Clostridium difficile infection, deadly diarrhea that causes at least 250,000 infections and 14,000 deaths each year in hospitalized patients in the United States, the CDC said.

Decreasing the use of the antibiotics that most often lead to Clostridium difficile infection by 30 percent could lead to 26 percent fewer of these deadly diarrhea infections, said the CDC.

These antibiotics include fluoroquinolones, -lactams with - lactamase inhibitors, and extended-spectrum cephalosporins.

To help hospitals improve antibiotic prescribing practices, the CDC said it has developed practical tools that include seven key elements, such as requiring automatic reassessment of prescriptions within 48 hours to make sure that the drug choice is appropriate.